We present near-infrared spectroscopic observations of SN 1987A covering the
period 1358 to 3158 days post-explosion. This is the first time that IR spectra
of a supernova have been obtained to such late epochs. The spectra comprise
emission from both the ejecta and the bright, ring-shaped circumstellar medium
(CSM). The most prominent CSM emission lines are recombination lines of H I and
He I, and forbidden lines of [S III] and [Fe II]. The ejecta spectra include
allowed lines of H I, He I and Na I and forbidden lines of [Si I], [Fe I], [Fe
II], and possibly [S I]. The intensity ratios and widths of the H I ejecta
lines are consistent with a low-temperature Case B recombination spectrum
arising from non-thermal ionisation/excitation in an extended, adiabatically
cooled H-envelope, as predicted by several authors. The slow decline of the
ejecta forbidden lines, especially those of [Si I], indicates that pure
non-thermal excitation was taking place, driven increasingly by the decay of
44Ti. The ejecta iron exhibits particularly high velocities (4000-4500 km/s),
supporting scenarios where fast radioactive nickel is created and ejected just
after the core-bounce. In addition, the ejecta lines continue to exhibit
blueshifts with values about -200 km/s to -800 km/s to at least day 2000. These
blueshifts, which first appeared around day 600, probably indicate that very
dense concentrations of dust persist in the ejecta, although an alternative
explanation of asymmetry in the excitation conditions is not ruled out.Comment: 18 pages, 8 figures, uses mn.sty, MNRAS, in pres